Probably the most serious cause of accelerated deterioration of the integrity of a reinforced concrete deck is corrosion of the reinforcing steel. Ground Penetrating Radar (GPR) is a tool that can identify quantities and locations of corrosion conditions that will cause reinforcement to corrode, concrete integrity and chemistry to change, and damage (such as cracking, delamination or spalling) to concrete. The sole variable considered in using GPR to assess the quality of the concrete is signal attenuation. A low amplitude reflected signal is interpreted to mean poor quality concrete. Conversely, relatively low signal loss is considered to mean the concrete quality is good.

Rii was subcontracted by Lochner, based in East Hartford, CT, to use GPR services to survey the reinforced concrete slabs of the five-level Stamford Transportation Center Parking Garage. The decision to close the garage was made by the Connecticut DOT for the safety of the public after pieces of falling concrete were discovered. The goal of the survey was to locate and map the extent and severity of the deterioration in the concrete slabs through the overlay wearing surface.

GPR was used to collect data related to the condition of the concrete decks of the five-story parking garage. The decks were scanned at intervals of 2-inch longitudinally and 2-foot transversely for the entire length and width of the concrete slabs. Due to the emergency alert, the GPR data was completed in 21 hours using a ground-coupled high resolution antenna with a center frequency of 1500 MHz.

A visual inspection was performed in conjunction with the GPR survey. This investigation was used to confirm the results generated by GPR and was useful in the analysis and interpretation of the GPR data. The visual inspection found that all the elevated deck slabs exhibited, in varying extent, deterioration in the form of delamination, scaling, spalling, and corroded exposed rebar. The floors of the slabs were also evaluated. It was observed that isolated areas of spalling and delamination with exposed rebar were present throughout the deck floors. Multiple areas of map cracking with efflorescence as a result of moisture penetration were also found during this inspection.

The GPR survey found that the percentage of advanced levels of concrete deterioration for the five decks of the parking garage ranged from 7.9% to 22.2% (red and orange colors on the maps). Orange and red colors on the maps indicated advanced state of deterioration, while the yellow color indicated an early age state of deterioration. To the discretion of Lochner Engineers, yellow spots can be considered as defective concrete as well and can be added to the orange and red spots to make the percentage of deterioration for the five slabs range from 17.1% to 41.1%. The GPR results were in close agreement with the results of the visual inspection of the parking garage.

Rii mobilized their team of engineers to complete and submit the report immediately after the field survey was completed, so the department of transportation could determine the next course of actions. The report highlighted the poor state of the decks that continue to exhibit signs of accelerated concrete delamination. It included scaled plan view maps of the slabs showing the location and size of the delamination and defective concrete. It also provided the total percent deterioration of the slab areas, and estimated square footage of the deteriorated concrete.